Automatic baud rate detection (autobaud) is used in current communication products such as modems, printers and plotters to facilitate the proper synchronization of transmission rates to send information such as data over a communication medium. It is necessary to synchronize the sending and receiving data transmission devices in order to assure that the information is correctly interpreted and no errors are incurred in the transmission. Baud is a communication term corresponding to bits per second (bps). Thus, 4,800 baud is 4,800 bits per second over the communication medium.
Two early examples of autobaud communication circuitry are found in U.S. Pat. No. 3,626,306 to Puckette and U.S. Pat. No. 3,696,338 to Preiss. These patents discuss techniques for synchronizing data transmissions employing the recognition of zero slope points of the received analog waveform and varying the transmission rates based on the presence or absence of information. Neither of these early techniques is as accurate or flexible as the present invention.
To date, the most common method for automatically determining the baud rate of a serial data stream has been to measure the duration of the start bit. This mechanism depends on the user device sending a specified character (autobaud character) as the first character of the session. This character typically has the characteristic of its Least Significant Bit (LSB) being a one (mark).
Technique of prior art approaches measures the duration of the start bit of the autobaud character to determine the baud rate of the transmitted information. However, industry standards for the timing of bits in a serial data stream are imprecise so that the width of the start bit may vary as much as fifty percent. This imprecision combined with the possibility of an erroneous character being sent instead of the correct autobaud character often cause incorrect baud rates to be selected.
Techniques using a start bit in an autobaud character are exemplified in U.S. Pat. Nos. 4,225,960 to Masters; 4,270,205 and 4,390,947 to Deshon; 4,418,322 to Chang; 4,468,752 to Chatham; 4,597,089 to Motley et al.; 4,663,766 to Bremer; 4,748,625 to Krause et al.; and 4,761,800 to Lese et al. This technique provides an accurate determination of the transmission rate, but it requires inflexible data information in each transaction. Techniques using measurement of the start bit duration place unnecessary performance constraints on the communication system and limit the accuracy and flexibility of the transmission medium.